“We’ll stop acting like adults if you stop acting like children,” students told crowds gathered in Garema Place for the “School Strike 4 Climate” rally on Friday.

We’re skipping school today to do some teaching, we’re teaching politicians about science. We’re teaching them that coal causes climate change. We’re teaching them what happens if they continue to do nothing.”

Organisers estimate 150,000 Australian students flocked to 50 rallies across Australia on Friday, part of a global movement spanning more than 100 countries that began in Sweden last year with teen activist Greta Thunberg.

Roads were closed off in parts of Civic as crowds marched to Glebe Park, holding high home-made signs declaring “Don’t burn our future” and “I can’t go to school today, I’m saving the planet”.

While the first school climate strike in November last year drew attention, this time around students wanted action. They came prepared with a list of demands (which they chanted down the line of Prime Minister Scott Morrison’s personal phone in Canberra) – an end to new fossil fuel projects, including the controversial Adani coal mine, and a shift to 100 per cent renewable energy in Australia by 2030.

George, 10, explained why he chose to skip school as he waited for a squadron of classmates cycling over from North Ainslie Primary.

“The earth is warming up and if adults aren’t going to do something about it, we sure are,” he said.

“We can’t vote so this is our vote.”

Parents, grandparents, activists and academics also joined the march, including Australia’s former chief scientist Penny Sackett. The reality is that the approach taken by adults so far isn’t working,” Professor Sackett said.

“School children striking around the world may be the beginning of a social movement more effective than 25 years of UN climate summits.”

Fourteen-year-old Maanha Manzur was one of about a dozen student organisers behind the event, coordinating security, land permits and public liability insurance in between classes.

She said the ACT turnout had greatly outstripped the first strike, which saw about 500 students brave the rain outside Parliament House. More than 3500 people poured into the city for Friday’s rally, she said, and at least 2000 of them were students.

Some said they had defied their schools by attending but many said they had been supported to head along, with parental permission. Most scoffed at criticism from federal ministers, including the prime minister’s calls for students to focus on learning not activism.

“Maybe they should actually do their job if they don’t want us striking,” one student said.

“We’re here because we’re almost out of time.”

Also among the crowd were federal candidates Tim Hollo from The Greens and Labor’s Alicia Payne, as well as ACT Minister for Climate Change Shane Rattenbury. Mr Rattenbury said he was inspired by the strikers and suggested those still denying the science of global warming should go back to school themselves.

Education Minister Yvette Berry also backed the protest as “learning in itself” and said students would not be penalised for attending.

But shadow education spokeswoman Elizabeth Lee questioned who was really behind the strike and suggested skipping school was not the best way for students to get their point across.

“I would hate for them to have been used as a political pawn in a matter as serious as climate change,” she said.

On Friday afternoon, students shrugged off the suggestion, collapsing gratefully in the shade of Glebe Park after months of hard work.

“We do have our own minds,” one laughed.

“But it’s motivating to see so many people behind us, even my grandma’s here.”